Angler Education

The Lateral Line: Subtle Water Movements

The exact function of the lateral line and what it senses have long been shrouded in mystery. It has been assigned functions as various as detecting light, touch, and chemicals to the secretion of slime.

In past articles I've written about the bass' skin, and it is sensitive to two key elements for a bass' behaviorﾗtemperature and pH. It is especially sensitive to changes in these two parameters that cause the fish to react, where to live and where to move for more comfort and perhaps more food.

But let's get more specific and explore a major sensory element found with the skin: the lateral line.

It wasn't until the 1960s that scientists discovered fish use their lateral lines to detect water motion. This motion can be generated by the fish itself, water currents or by some external moving object. And this motion detection ultimately results in finding food.

A bass' lateral line system is comprised of two major subdivisions: one on the head and one along the body trunk. These two subdivisions differ not only in structure and in the nerve branches that supply them but also in how the bass uses them.

The first is a network of highly branched canals spread over the head and face of the bass, especially around the eyes, across the top of the head, beneath the jaw, and along the front portion of the gill cover.

The other consists of a single long canal of specialized scales on either side of the trunk, starting just behind the eyes and extending to the tail. Along the canals are regularly spaced openings, or pores. The pores allow water to flow freely in either direction along the canals' length.

The lateral line seems adept at detecting the subtle movements of biological sources (small prey fish, for example) located some distance away. The fish feels out distant objects by interpreting how they disrupt the intervening water.

While the lateral line helps detect the next meal, more senses come into play before the actual dining. Visual confirmation with the eyes moves the bass closer. Once the prey is within a couple of body lengths, the lateral line is deadly accurate but other senses still play key roles. Often the bass simply moves in guided by its vision to the target, never hesitating. The bass makes the decision and goes for it. Other times the bass might pause during the final few feet of approach.

Now the bass is using the lateral line along the trunk as well as the eyes and the lateral line in the head to determine if the object is something to eat.